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Hi all I am putting back together a 1969 t150 engine at the moment I have just put the gearbox back in and was going to re fit clutch etc but noticed there is ne seal between hire gear shaft and main shaft and not a lot of space for one. The sprocket retaining nut is just a nut, I looked on later parts lists and they allowed for an O ring. Is this a later mod. or do I have something missing or a wrong part and could I retro fit an O ring with later nut. At the end of the day I don’t like the idea of leaving part of the main shaft exposed next to where the chain is running. If anyone can help I would be most grateful. Thanks Toby (Richard)

I have just stripped my 1974 T150V to rectify an oil leak from the gearbox main shaft seal area. There is a groove ground into the main shaft to accommodate the ‘O’ you mentioned. Usually the shaft is sealed with hydroseal and an ‘O’ ring. My bike had no sealant and the tab washer was fitted with the splines toward the gearbox which lacerated the ‘O’ ring. I have re-built it using silicone to seal the splined main shaft. If your main shaft does not have a ‘O’ ring groove, I don’t see how you can fit one, a liberal coating of silicone on the splines should be enough to keep the oil in the gearbox, hope this helps!

Hi, the early 4 speeds did not have a seal on the mainshaft and relied on the clearance on the bush and (I think) machine grooves in the bush. So as the shaft turns oil is forced back into box along the grooves. Now later Triumph used a shield sprocket nut which housed an ‘o’ seal in an attempt to hold back any oil that may have escaped past the bush. Note splines have to be sealed too using a loctite shaft sealer. It is what it is, and leaks aren’t helped by putting bike on side stand and overfilled gearboxes

So yes, you can retro fit the later shield nut and ‘o’ ring.

5 speeds had a mechanical seal on mainshaft behind bush. and used an ‘o’ seal behind sprocket nut to seal splines.